This invention relates to a process for catalytically isomerizing 1,5-dimethylnaphthalene (1,5-DMN), 1,6-dimethylnaphthalene (1,6-DMN), 2,6-dimethylnaphthalene (2,6-DMN), or some mixture thereof, at an elevated temperature in the gas-phase selectively to a mixture of 1,5-DMN, 1,6-DMN, and 2,6-DMN (the 2,6-DMN triad) in which the amount of each of the 2,6-DMN triad isomers closely approximates the equilibrium concentration calculated for the temperature at which the isomerization is carried out, and production of outside-the-2,6-DMN-triad products is minimized. More particularly, this invention relates to an elevated temperature, molecular-sieve-catalyst-composition catalyzed, gas-phase isomerization process wherein a feed containing one or more of the 2,6-DMN triad isomers is isomerized over a lower acidity, molecular-sieve-based catalyst composition to a reaction product containing essentially 2,6-DMN triad isomers and wherein the amounts of the respective 2,6-DMN triad isomers in the reaction product are close to their equilibrium values calculated for the temperature of isomerization.
The ten isomers of dimethylnaphthalene can be conveniently divided into four groups when discussing isomerization. These are the 2,6-DMN triad (1,5-DMN; 1,6-DMN; 2,6-DMN), the 2,7-DMN triad (2,7-DMN; 1,7-DMN; 1,8-DMN), the 2,3-DMN triad (2,3-DMN; 1,3-DMN; 1,4-DMN), and 1,2-DMN. For various theoretical reasons, forming other members of a triad from a same-triad isomer requires less energy than forming outside-the-triad isomers. Advantage of this situation can be utilized if the appropriate isomerization catalyst can be found, since commercially a single isomer of DMN is often required in high isomeric purity. For example, one method of preparation of 2,6-DMN, an alkenylation process, forms 1,5-DMN as an intermediate and uses as a final step, the isomerization of 1,5-DMN to 2,6-DMN after which the 2,6-DMN is separated and the residue recycled to the isomerization step. In such a process, it is necessary to produce the 2,6-isomer with as little outside-the-triad products as possible and beneficial to reach, in each pass through the isomerization step, the equilibrium concentration of 2,6-DMN calculated for the temperature of isomerization. This insures a slow buildup of unwanted products in the recycle system and improves the per pass production of 2,6-DMN. A number of different catalysts have been suggested for the above alkenylation process isomerization step, but no one is completely satisfactory in that either too much outside-the-triad products are formed per pass or near equilibrium amounts of within-the-triad isomers are not formed.
A substantial literature exists for the isomerization of DMN and within it attention has been given to isomerization of both pure DMN isomers and DMN isomer mixtures. Attention has also been directed to carrying out the isomerization in both gas and liquid phases. An example of a study of the liquid-phase isomerization of DMN is found in J. Org. Chem. 29, 2939 (1964) where an HF/BF.sub.3 mixture was used as a catalyst. Japanese Kokai No. 50-117757 (1975) teaches liquid-phase isomerization using a Y-type zeolite catalyst and claims superiority of the liquid-phase process over the gas-phase process. Gas-phase isomerization of 1,5-DMN, 1,6-DMN or a mixture of the two (isomerization within the 2,6-triad) is set forth in U.S. Pat. No. 3,798,280 wherein the catalysts taught are silica-alumina, alumina-boria or a zeolite. Isomerization of DMN to 2,6-DMN and 2,7-DMN (isomerization outside of a single triad) is taught in Ger. Offen. No. 2,243,005. The catalyst used was an alumina/silica molecular sieve. None of these processes and their attendant catalysts, particularly the ones for gas-phase isomerization within the 2,6-DMN isomer triad, are particularly satisfactory in that they do not allow rapid and essentially complete approach to the equilibrium isomer concentrations at the isomerization temperature (particularly the 2,6-DMN concentration) while simultaneously keeping the concentrations of the members of other isomer triads and cracked products at a low enough level to make the necessary recycle and 2,6-DMN separation efficient.
Now catalyst compositions have been found which are able to produce near equilibrium amounts of within-the-triad isomers at reasonable catalyst composition space velocities and also produce low amounts of outside-the-triad products, both other DMN isomers and cracked products.